Depends little bit, what you want. I don't like irssi too much, but for something over gnu screen or tmux this is pretty usable for longer sessions, collecting channel conversations (so, poor man bouncer + client setup). HexChat is decent Linux or Windows client. When I used Windows, I liked mIRC, this was (and probably is) very versatile. I have seen customisations where you don't even feel that mIRC is used.